The present invention relates to a selectively operating, chemical gas sensor which includes as its main active element a capacitor with a dielectric material in between the electrodes which layer is constituted by a gas permeable layer.
Many instances of automated processes require the utilization of many relatively simple, small and economic measuring transducers and associated devices. This is particularly important concerning the measurement of contaminants and contaminate emissions. The demand for equipment along this line is in fact increasing with the growing import of ecologically safe manufacturing and other facilities. In connection therewith certain transducers have been developed that acquire a chemical quantity, such as the concentration of a particular material, and provide a measuring result provided of course it is physically possible to obtain and actually acquire a reasonable and reasonably reliable measuring quantity. The measured item is converted so to speak by the transducer into an electrical signal such as a current or an electrical voltage (or both).
Transducers of the kind referred to above are generally called chemical sensors. They of course involve an indirect measuring method which on the other hand carries with it the problem of selectivity and transverse sensitivity. As compared with physical sensors these particular parameters of chemical sensors are comparatively low.
Generally speaking chemical sensors are constructed to have some form of gas sensitive layer and a carrying transducer element for other components and which together constitute the transducer proper. The electrical converting element may be in an oscillating quartz. Moreover, the device will include interdigital capacitors, FET-s and the like. The transducer proper (other than the sensitive layer) is not chemically sensitive but the transducer has an input structure including the gas sensitive layer that converts the chemical parameter into another physical quantity which in turn is then converted into an electrical parameter. The effect of course is basically an interaction between the environment to which the device is exposed on one hand and the aforementioned chemically sensitive layer.
Electrically insulating gas selective coatings are provided on microstructure element in the following fashion. So called organically modified silicates (also ormosiles) are used as a selectively active, adsorber structure and the coating is comprised of an amorphic skeleton to support a molecular frame under utilization of siloxane compounds (Si--O--Si) which acts in an analogous fashion to anargonic skeleton silicates. The specifically effective adsorption of gas molecules is provided through functionally organic groups. The physical-chemical measuring effect now resides in the change of the dielectric constant in and of whatever layer is used on account of the adsorption process involving whatever is being picked up from the environment. However, it is apparent that adsorption obtains only in the surface and therefore the sensitivity of such an ormosile layering is quite low.
The manufacture of monomolecular coatings on microstructures components use the following technology developed by Langmuir and Blodgett. A film is provided (floating) on top of a water surface, the film being constituted by elongated molecules and the water is used just as carrier. This film is then deposited on the microstructure surface and construction element. First tests here were made under utilization of phtalocyanines. This material has a known adsorption capability as far as NO molecules are concerned. However, through variation of the central molecule in this phthalocyanine complex one can render the sensor sensitive to other molecules.
Still another method involves tin oxide layers which are provided by operation of vacuum depositing and cathode sputtering. This concept deposits thin polycrystalline tin oxide layers on an insulating carrier. Suitable metallizing is provided in addition for obtaining the requisite electrodes. These elements are heated up to 300 to 400 degrees C in order to obtain an interaction between gas of the environment and the particular sensitive layer. Under participation of reducing gases the layer modifies its conductivity as a function of gas concentration. However, a significant problem of this device arise from its inadequate selectivity as well as lack of stability of the layer involved.
Independently from the foregoing tests have been conducted for using zeolite as a filter for coating of FET-s. Their effect is supposed to be based on the different and selective permeability of the zeolite for differing gases. However, significant problems have resulted on account of the diffusion of gases through so called secondary pores that exist right at the gates of the FET. This diffusion in fact eliminates selective operation of these devices.